Estimates of the hip joint centers (HJC) are usually based on predictions from anatomical landmarks and anthropometrical measures such as pelvic breadth. There is concern regarding the relevance of this approach in patients characterized by obesity or pelvic deformity. Recent investigations have explored estimating HJCs using the functional method. Current literature often fails to disclose the form of the algorithm used in these tests, although three distinct forms exist. In addition, previous research has ignored the interaction between the algorithms, the portion of sphere surface represented by the data, the nature and magnitude of noise contained in the data, and the effect of different marker placement strategies. The goal of this project is to understand these interactions in order to know when clinical implementation of the functional method is appropriate and when it is likely to produce erroneous results. The specific aims of his project are: 1) to model the area of motion and radial perturbation values as a function of the error associated with the functional algorithms, 2) to determine the effect of area of motion on HJC estimates measured under non weight-bearing conditions, 3) to determine the effect of marker placement on radial perturbations and HJC estimates under non weight-bearing conditions, 4) to compare the functional algorithms to estimates from prediction methods and with known HJC locations based on ultrasound and DEXA imaging, and 5) to determine the characteristics of functional algorithms and marker placement strategies on HJC estimates measured during weight-bearing. In the computer modeling component of this study, the range of hip flex/ext, abd/add, and random radial noise will be systematically altered in order to produce maps of the centroid displacement. The maps calculated from these simulations will be used to establish theoretical constraints for use of the algorithms in clinic. In the clinical component of the study, the standard for HJC location will be determined with DEXA and ultrasound. Motion analysis data collected during standing and walking trials will be used to reconstruct HJCs from 1) different algorithms, 2) different virtual knee marker reconstruction methods, and 3) delimited amounts of sphere surface. The characteristics of marker noise associated with different placement strategies will be analyzed. Resulting HJCs will be compared to the standard to determine the interactions of each variable, and the acceptable range of values from each variable that will produce HJCs within 1 cm of the standard.